The present invention relates to a three-phase electrical power measurement system. More specifically the present invention relates to a three-phase electrical power sub-metering system including three transformers where the measurement device is enclosed within a housing surrounding one of the transformers.
Electrical power is provided to many devices, such as large motors, by three separate cables, each of which supplies a single phase of three phase-power. In an ideal system, each of the phases within the respective cable has a phase angle which is generally 120 degrees apart from the other phases. Accordingly, the total power flowing through the three cables to a three-phase load (or from a three-phase generator) is:
p(t)3xe2x88x92phase=VpIp*[cos(2wt+a+xcex2)+cos(2wt+a+xcex2xe2x88x92240)+cos(2wt+a+xcex2Pxe2x88x92240)+cos(2wt+a+xcex2xe2x88x92480)]+3*VpIp*cosxcfx86,
where Vp and Ip represent the root-mean-square values of the phase voltages and phase currents.
Although there exists numerous devices suitable to measure the power flowing through a single conductor, these devices are not suitable to measure power flowing through multiple conductors. For example, one device for measuring power through a single conductor is produced by Veris Industries, Inc. of Portland, Ore. Veris Industries, Inc. markets a single phase power measurement device under the name KT 6300 that includes a split core transformer that encircles a cable to sense the current flowing therein. The KT 6300 also includes multiple wire leads that are connected to the one or more cables to sense the voltage therein. A measuring circuit enclosed within the housing of the transformer calculates the power flowing through the cable. Unfortunately, the KT 6300 is not capable of measuring the power usage of three-phase power systems.
An electrical power utility measures the power usage of each of its customers using a power revenue meter (normally on the exterior of each customer""s building). The power revenue meter electrically interconnects the secondary service of the utility with the primary service of the customer. The electrical power used by the customer is measured by the power revenue meter and the customer is billed periodically.
The power revenue meter is normally a glass meter with a spinning disc that rotates proportionally to power usage. To install such a power revenue meter in new construction the customer routes a first three-phase cable (three separate conductors, each of which carries a single phase) from a customer""s power box to a power revenue meter base for power returned from the customer to the utility. The power box is normally located within a customer""s building and encloses a panel with circuit breakers for distribution of the electrical power to different electrical loads of the customer. Such loads may, for example, include lighting, motors, air conditioning systems, and pumps. A power revenue meter base is installed on the exterior of the building and the first cable is connected thereto. A second three-phase cable is connected to and routed from the power revenue meter base to the power box for supplying power used by the customer. After the two three-phase cables are properly installed, a power revenue meter is installed in the power revenue meter base. The installation of the three-phase cables and the power revenue meter (including its base) is labor intensive and incurs substantial expense.
The expense associated with installing the power revenue meter in new construction is normally included in the total construction cost.
Installation of the power revenue meter in an existing building is substantially more expensive than installation of the power revenue meter in new construction. In existing buildings, at least one hole needs to be drilled though the wall and conduit routed between the power box on the interior of the building and the power revenue meter on the exterior of the building. Also, suitable interior wall space must be located to mount the power box in a location near the power revenue meter. If suitable wall space is not available nearby then an excessive length of conduit must be installed or devices located on the nearby suitable portion of the wall need to be relocated, both of which are time intensive and expensive.
There are numerous occasions in which a customer may wish to install additional power revenue meters. For example, customers may wish to monitor power usage of particular loads using additional power revenue meters. Many companies desire to allocate their electrical power usage based on power usage by individual departments. By using multiple power revenue meters the expense for electrical power usage can be allocated and monitored at the department level. In this manner each department is responsible for payment of their own electrical power usage.
Shopping malls and marinas are examples of customers that often need to install additional power revenue meters for tenant sub-metering. In these cases, each tenant""s individual power usage is individually billed to that particular tenant, as opposed to merely guessing what portion of the total power usage is attributable to each tenant. Tenant sub-metering is important when there are significant differences between the amounts of power usage by different tenants.
Another example of a customer that may need to install additional power revenue meters are universities or other multiple building institutions that desire to determine where electrical power is being wasted because many buildings have antiquated electrical systems. Monitoring power usage on an individual building basis permits the customer to renovate those portions of the institution where the resulting cost savings will pay for, at least in part, the renovations.
Sometimes multiple power revenue meters are used to isolate use of particular systems. For example, cooling systems use a substantial amount of electrical power so there is a need for installing additional power revenue meters for optimizing the cooling systems to reduce electrical power usage. When redesigning cooling systems and adjusting electrical usage of different portions of existing systems there is a trade off between the electrical power consumed by the pumps which vary the fluid flow and the electrical power consumed by the fans. A proper balance between the electrical power usage of the pumps and fans may reduce the overall power usage.
Unfortunately for most systems, such as departmental billing, tenant sub-metering, multiple building institutions, and cooling systems, the expense associated with installing additional power revenue meters does not outweigh the potential benefits to be derived therefrom.
In contrast to installing additional utility power revenue meters, a power sub-metering system may be used to provide sub-metering capability. Sub-metering involves measuring the power delivered from a customer""s power box to a particular device. As opposed to installation of additional power revenue meters that are monitored by the utility company, a customer using sub-metering receives a single bill from the utility for each power revenue meter but is able to allocate the utility bill from each power revenue meter among its different uses using information provided by the sub-metering system.
A sub-metering system generally includes a separate transformer installed on each respective cable of the three cables of a three-phase system within a customer""s power box containing the electrical panel. Each transformer senses a changing current within a respective cable and produces an output voltage or current proportional to the changing current. A measuring circuit is electrically connected to the three transformers and receives each of the transformer output voltages or currents. The measuring circuit is also electrically connected to the three cables by voltage xe2x80x9ctapsxe2x80x9d to measure the voltage therein. The voltage xe2x80x9ctapxe2x80x9d measurements are preferably obtained by an electrical connection to the interface between each phase of the respective cable and the panel. The measuring circuit calculates the power usage of the respective three phases using the output voltages from the transformers and the voltages sensed by the three voltage xe2x80x9ctaps.xe2x80x9d
For safety reasons electrical building codes prohibit the installation of the measuring circuit in the power box, such as the power box containing the electrical panel with high voltage conductors. In order to calculate the electrical power usage, a separate measuring box must be purchased and installed in a location proximate the power box. A conduit is installed to interconnect the measuring box and the power box. The measuring circuit is then installed within the measuring box. The installation of the separate measuring box is time consuming, labor intensive, and expensive. In addition, if suitable wall space is not available for the measuring box, then relocation of other devices on the wall may be necessary at added expense. Also, each of the transformers has a pair of wires extending therefrom which are routed through the conduit to the measuring circuit in the measuring box, and a set of three wires connected to the voltage xe2x80x9ctapsxe2x80x9d are likewise routed through the conduit to the measuring circuit in the measuring box. Accordingly, at least nine wires need to be routed between the power box and the measuring box. Installers of the sub-metering system have a tendency to become confused as to where each wire originated and frequently connect the wires improperly. If the wires are improperly connected then the measuring circuit will improperly calculate the power usage. Also, it is time consuming to verify which wires are connected to which cables within the two separate boxes.
What is desired, therefore, is a cost effective sub-metering system for a three-phase system that is inexpensive, is quick to install, does not require installation of additional enclosures, and is not prone to improper connections.
The present invention overcomes the aforementioned drawbacks of the prior art by providing a measurement system that includes a first transformer enclosed within a first housing and magnetically linked to a first cable where the first transformer senses changing current within the first cable and in response produces a first output voltage. A second transformer is enclosed within a second housing and magnetically linked to a second cable where the second transformer senses changing current within the second cable and in response produces a second output voltage. A third transformer is enclosed within a third housing and magnetically linked to a third cable where third transformer senses a changing current within the third cable and in response produces a third output voltage. A measurement circuit is electrically connected to the combination of the first transformer to receive a first input signal representative of the first output voltage, the second transformer to receive a second input signal representative of the second output voltage, and the third transformer to receive a third input signal representative of the third output voltage. The measurement circuit calculates an output value representative of electrical power within the first, second, and third cables in response to receiving the first, second, and third input signals. The measurement circuit is enclosed within at least one of the first housing, the second housing, and the third housing.
One of the principal advantages of the measuring system is that the measuring circuit is enclosed within one of the housings. Electrical building codes permit the measuring circuit to then be located within a power box. By locating all three transformers and the measuring circuit within the power box, while being in compliance with the electrical building codes, there is no need to install an additional measuring box and conduit thereto. Without the need to purchase and install an additional measuring box, there is no need to relocate any devices supported by the wall that would have been otherwise prevented installation of the measuring box. The likelihood of improperly connecting the transformers and wires with the proper cables is small because all the wires are contained within a single enclosure, namely the power box, and not obscured from view by passing through a conduit.
The foregoing and other objectives, features, and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawings.